Compositions and methods for planarizing or polishing the surface of a substrate are well known in the art. Polishing compositions (also known as polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the slurry composition. Typical abrasive materials include silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. U.S. Pat. No. 5,527,423, for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface with a polishing slurry comprising high purity fine metal oxide particles in an aqueous medium. Alternatively, the abrasive material may be incorporated into the polishing pad. U.S. Pat. No. 5,489,233 discloses the use of polishing pads having a surface texture or pattern, and U.S. Pat. No. 5,958,794 discloses a fixed abrasive polishing pad.
Conventional polishing systems and polishing methods typically are not entirely satisfactory at planarizing microelectronic devices. In particular, polishing compositions and polishing pads can have less than desirable polishing rates, and their use in the chemical-mechanical polishing of microelectronic devices can result in poor surface quality. Because the performance of a microelectronic device is directly associated with the planarity of its surface, it is crucial to use a polishing composition and method that results in a high polishing efficiency, uniformity, and removal rate and leaves a high quality polish with minimal surface defects.
The difficulty in creating an effective polishing system for microelectronic devices stems from the complexity of the typical microelectronic device. Microelectronic devices, such as semiconductor wafers or memory devices, typically are composed of a substrate, on which a plurality of transistors has been formed. Integrated circuits are chemically and physically connected into a substrate by patterning regions in the substrate and layers on the substrate. To produce an operable semiconductor wafer or memory device and to maximize the yield, performance, and reliability of the wafer or device, it is desirable to polish select surfaces of the wafer or device without adversely affecting underlying structures or topography. In fact, various problems in semiconductor and/or memory fabrication can occur if the process steps are not performed on surfaces that are adequately planarized.
Various metals and metal alloys have been used to form electrical connections between devices, including titanium, titanium nitride, aluminum-copper, aluminum-silicon, copper, tungsten, platinum, platinum-tungsten, platinum-tin, ruthenium, and combinations thereof. Noble metals present a particular challenge in that they are mechanically hard and chemically resistant, making them difficult to remove efficiently through chemical-mechanical polishing.
The following patents and published patent applications disclose polishing compositions for noble metals. U.S. Pat. No. 5,691,219 discloses a semiconductor memory device comprising a noble metal conductive layer and a polishing composition comprising a halo-compound for polishing the noble metal. U.S. Pat. No. 6,274,063 discloses polishing compositions for nickel substrates comprising a chemical etchant (e.g., aluminum nitrate), abrasive particles, and an oxidizer. U.S. Pat. No. 6,290,736 discloses a chemically active polishing composition for noble metals comprising an abrasive and a halogen in basic aqueous solution. JP 63096599 A2 discloses a method of dissolving metallic ruthenium. JP 11121411 A2 discloses a polishing composition for platinum group metals (e.g., Ru, Pt) comprising fine particles of an oxide of the platinum group metal. JP 01270512 A2 discloses a dissolving solution for noble metals comprising hydrogen peroxide, alkali cyanide, and phosphate ion and/or borate ion. WO 00/77107 A1 discloses a polishing composition for noble metals (e.g., Ru, Rh, Pd, Os, Ir, Pt) comprising an abrasive, a liquid carrier, an oxidizer, and a polishing additive that can include EDTA, nitrogen-containing macrocycles (e.g., tetraazacyclotetradecanes), crown ethers, halides, cyanides, citric acid, phosphines, and phosphonates. WO 01/44396 A1 discloses a polishing composition for noble metals comprising sulfur-containing compounds, abrasive particles, and water-soluble organic additives which purportedly improve the dispersion of the abrasive particles and enhance metal removal rates and selectivity.
While the aforementioned polishing compositions may be capable of polishing metals (e.g. noble metals) at desirable rates, they are not very effective at polishing metals in an oxidized form, such as iridium oxide. However, the use of such oxidized metals is becoming increasingly popular. For example, oxidized metals (i.e., metals in an oxidized form) have been and continue to be used in integrated circuits, memory devices (e.g., ferroelectric random-access memory), micro-electrical-mechanical systems (MEMS), and micro-electrical-optical systems (MEOS)). More specifically, iridium oxide (IrO2) currently is used as a barrier/adhesion layer in certain ferroelectric random-access memory (FeRAM) capacitor schemes. Despite the prevalence of such uses, very few commercially available polishing systems and known polishing methods can be used to effectively and efficiently polish a substrate comprising an oxidized metal (i.e., a metal in an oxidized form).
Accordingly, a need remains for polishing systems and polishing methods that will exhibit desirable planarization efficiency and/or removal rate during the polishing and planarization of substrates comprising a metal in an oxidized form. Improved polishing systems and methods are particularly needed for the polishing of substrates comprising chemically stable and mechanically hard noble metals in an oxidized form. The invention provides such a polishing system and polishing method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.